Fuel question

My 97 Mustang regulary takes the regular octane fuel but...occasionally i put mid-level gas in it...is that bad for the engine??....some of the rumours i heard was putting mid-level gas in your car gives you slightly better gas mileage and it makes your engine perform slightly better.....is this shit false and am i un-knowingly damaging my engine??

Mid-grade does in fact give you better mileage. But not something you'll see in one gallon of fuel. You'll notice may a 5% increase MPG. Depending on current mileage that you're getting, this may very well be worth it. Mid-grade also enhances performance, but not a difference you will notice really. It's not going to damage your engine to switch from mid-grade to regular unleaded and back again. As a personal suggestion though, you should decide which you prefer, and stick to that one. Best to judge from mileage, not "performance." Just my $0.02, take or leave it.